1. Field of the Invention
This invention relates to the field of thickness measurement systems, and particularly to the use of laser ultrasonics to determine glass thickness.
2. Description of the Related Art
To avoid breakage during shipping and handling, the walls of glass products such as bottles and beakers must typically meet a minimum thickness specification. To determine adherence to this specification, the glass thickness must be measured. Glass products of this sort are usually conveyed through a factory in large numbers. To avoid stopping or unduly slowing their movement, such monitoring is preferably performed xe2x80x98in-linexe2x80x99, i.e., as the products travel along the conveyor, and without contacting the glass.
A conventional in-line measurement technique uses a laser system which determines glass thickness using triangulation. A laser beam is directed at the glass to be measured at an oblique angle to the surface. A portion of the beam reflects off the outer surface of the glass, while the remainder of the beam is transmitted through the outer surface; a portion of the transmitted beam is also reflected off the inner surface. The spatial displacement of the two reflected beams is measured, and the thickness of the glass determined from the displacement measurement by a simple geometric calculation.
Unfortunately, as one of the beams must pass back and forth through the glass, this technique works poorly on colored glass, or else requires the use of different laser types for different glass colors. This technique also requires the inner and outer surfaces to be largely parallel, or the accuracy of the technique is lost.
A system and method for measuring the thickness of glass is presented, which provides in-line, non-contacting thickness measurements regardless of glass color, while avoiding the problems noted above. In addition, the degree of parallelism required is considerably less than that needed in the prior art.
The invention uses a laser-based ultrasound technique to determine glass thickness. An ultrasonic wave is induced between the surfaces of a region of glass. In one operating mode, the ultrasonic wave reflects back and forth between the surfaces repeatedly, building up a reverberation which causes both surfaces of the glass to move in and out at the same characteristic reverberation frequency. The surface motion is monitored to determine the characteristic reverberation frequency, which is inversely proportional to the thickness of the glass in the region of the ultrasonic wave.
In another operating mode, the ultrasonic wave comprises short, discrete pulses which reflect back and forth inside the glass sample. Each time the pulse arrives at a surface, transient surface motion is induced. The pulses (called xe2x80x98echoesxe2x80x99) arrive at equally spaced time intervals. The surface motion is monitored to determine the characteristic xe2x80x98time-of-flightxe2x80x99 (TOF) between successive pulses. The thickness of the glass in the region of the ultrasonic wave is directly proportional to the time-of-flight.
The ultrasonic wave is preferably induced with a pulsed xe2x80x98generationxe2x80x99 laser which produces a short duration pulse that illuminates a surface of the glass. The wavelength of the generation laser is chosen such that the pulse is largely absorbed near the illuminated surface or in the bulk of the material, causing a rapid thermal expansion which results in the creation of the ultrasonic wave. To avoid damaging the glass, the pulse duration, beam size, and energy characteristics of the generation laser must be carefully chosen. The surface motion induced by the ultrasonic wave is preferably detected with an interferometer system which utilizes a separate probe laser. The output of the interferometer is analyzed to determine the TOF or characteristic reverberation frequency of the surface motion.
Because the technique does not require the outputs of the generation or probe lasers to be transmitted through the glass, the glass need not be transparent. Furthermore, since the probe laser need not be reflected off the back surface (as in the prior art), there is no strict requirement that the glass surfaces be parallel; rather, the surfaces need only be sufficiently parallel to allow multiple reflections or reverberations of the ultrasonic wave in a local region. Finally, the present technique requires no contact with the glass, is non-destructive, and is suitable for in-line measurements.
An alternative method of detecting surface motion, useful when the glass is largely transparent with nearly parallel surfaces, utilizes the glass itself as an etalon. The intensity modulation produced by the resulting interference pattern is detected with one or more photocells, with the frequency of the modulation establishing the TOF or characteristic reverberation frequency needed to determine thickness.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.